Generally, in the electrical control of a device in a motor vehicle, a voltage from a battery (for example a 12 Volt or 24 Volt battery) is supplied to a sensor switch or to a driving circuit for the device whereby a detected signal or a feedback signal in this voltage level is obtained. This signal is transformed into a binary signal, for example, a 5 Volt binary signal so as to supply and operate a microprocessor for controlling the device. Therefore, it is required that a relatively high level signal voltage formed in accordance with the voltage from the battery is transformed into a relatively low level voltage signal for application to the microprocessor.
A prior art signal level transforming circuit is disclosed in Japanese Patent Laid-open No. 58(1983)-162124 published on Sept. 26, 1983. The basic structure of the signal level transforming circuit of this prior art reference is shown in FIG. 3. In FIG. 3, the signal level transforming circuit comprises a high voltage level signal input terminal VIN, a high voltage supply terminal VB connected with a 12 Volt battery (not shown), a low voltage supply terminal VCC connected with a low voltage source (not shown) for supplying a constant 5 Volt voltage, a ground terminal GND, a low voltage level signal output terminal VA, a first constant current circuit CCf, a low voltage clamping circuit LVc (R5, R6, Q3, D1), a high voltage clamping circuit HVc (R3, R4, Q4, Q5), a second constant current circuit CCs, and a comparing circuit VCP. FIGS. 4A and 4B show an input-output characteristic of the signal level transforming circuit.
The second constant current circuit CCs and the comparing circuit VCp form a hysteresis circuit A1. The hysteresis circuit A1 outputs a preset constant current I16 when a voltage at the signal output terminal VA reaches a standard value. A constant current IE6 flowing through a transistor Q6 in the first constant current circuit CCf is indicated as follows: IE6=IA (constant). Namely, any current larger than the constant current IE6 does not flow into the transistor Q6. Therefore, the constant current I16 flows into a resistor RA, so a current flowing through resistor RA is indicated as (IA-I16). An output voltage VA appearing at signal output terminal VA is indicated as follows: ##EQU1## Therefore, the output voltage VA increases by I16.times.RA. This voltage I16.times.RA is a hysteresis-up voltage Vo. The output voltage VA changes in response to an input voltage VIN because I16 is a constant current.
When the input voltage VIN drops to V2 from V3, the output voltage VA drops to VA3 in response to input voltage VIN due to the reason described above. When the output voltage VA drops to VA3, the supply of constant current I16 from transistor Q6 is stopped whereby the value of I16.times.RA=Vo changes to 0 (zero), the output voltage VA rapidly drops to VA=VIN-IA.times.RA=VA2. When the output voltage VA drops to VA1 from VA2, the output voltage VA drops in response to the decrease of input voltage VIN because IA=IE6 is constant in the above-described formula VA=VIN-IA.times.RA. In FIG. 4A, when the input voltage VIN drops to V2 from V3, the output voltage VA drops to VA3 from VA4 in response to the change of input voltage VIN. The output voltage VA is in direct proportion to the input voltage VIN, so the gradient of the straight line which indicates a relationship between input voltage VIN and output voltage VA is (VA4-VA3)/(VA4-VA3)=1/1, therefore, the hysteresis-up voltage Vo equals to a hysteresis width VH.
However, when the hysteresis-up voltage Vo equals the hysteresis width VH, a problem occurs as follows: It is that if the hysteresis-up voltage Vo increases extremely, the hysteresis width VH' expands as shown in FIG. 4B. In FIG. 4B, for example, if VA2 and VA3 are 0.8 Volt and 4 Volt respectively, the hysteresis-up voltage Vo is (VA3-VA2) -3.2 Volt. Therefore, as described above, the hysteresis width VH is also 3.2 Volt due to Vo=VH. If this situation occurs, there are not any problems with respect to the output signal voltage VA rapidly rising up to VA3 from VA2, however, when the output signal voltage VA drops to VA2 after it rises up to VA3, the distinction of the binary signal from the output signal voltage VA with respect to the input signal voltage VIN in this part cannot be certain because the output signal voltage VA drops in response to the input signal voltage VIN. Consequently, in this part, if the output signal voltage VA changes due to the change of input signal voltage VIN based on the change of battery voltage, there is a possibility that the microprocessor may make an error in its operation. Furthermore, in prior art signal level transforming circuit, there are defects in that the circuit structure is complex and the number of circuit components is large.